Jan Burse, created Sep 15. 2018
* Prolog code for finding a perfect number.
* A solution that did not make use of findall/3 and rem/2
* was posted Dec 16, 1988 on comp.lang.prolog by Thomas
* Sj|land. It was titled Christmas pleasure and contained
* an invitation to further dig into Euclids theorem.
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% between1(+Integer, +Integer, -Integer)
% sumlist1(+List, -Integer)
% perfect(+Integer, -Integer)